Electrical configuration for object detection system in a saw

ABSTRACT

An object detection system in a saw includes an electrically conductive plate positioned at a predetermined distance from the implement, a detection circuit comprising a transformer, and a single cable connecting first terminal and second terminals of the transformer. The single cable includes a first conductor electrically connected to the first terminal of the winding and to the electrically conductive plate, a second conductor electrically connected to the implement, and an electrical insulator positioned between the first conductor and the second conductor.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.15/060,656, filed on Mar. 4, 2016, which claims priority to (i) U.S.provisional application No. 62/131,977, filed on Mar. 12, 2015 and (ii)U.S. provisional application No. 62/132,004, filed on Mar. 12, 2015, theentire contents of which are hereby incorporated by reference herein.

CROSS REFERENCE

This application cross-references U.S. Pat. No. 9,914,239 to Laliberteet al., issued Mar. 13, 2018; U.S. Pat. No. 10,099,399 to Laliberte etal., issued Oct. 16, 2018; U.S. Pat. No. 10,105,863 to Ramaswamy et al.,issued Oct. 23, 2018; U.S. Pat. No. 10,189,098 to Laliberte et al.,issued Jan. 29, 2019; and copending U.S. application Ser. No.15/060,709, filed on Mar. 4, 2016, the entire contents of which arehereby incorporated by reference herein.

FIELD

This disclosure relates generally to power tools, and, morespecifically, to systems and methods for detecting contact between animplement and objects in a saw.

BACKGROUND

Detection or sensing systems have been developed for use with variouskinds of manufacturing equipment and power tools. Such detection systemsare operable to trigger a reaction device by detecting or sensing theproximity or contact of some appendage of an operator with some part ofthe equipment. For example, existing capacitive contact sensing systemsin table saws detect contact between the operator and the blade.

FIG. 1 depicts a prior art capacitive sensing based detection system 90that is incorporated with a table saw 1. The detection system 90 drivesan excitation voltage that is electrically coupled to a movable blade 22of the saw 1, and detects the current drawn from the blade 22. Theamplitude or phase of the detected current and/or excitation voltagechanges when the blade 22 comes into contact with an electricallyconductive object (such as an operator's hand, finger or other bodypart, as well as work pieces). The characteristics of the changes areused to trigger the operation of a reaction system 92. The reactionsystem 92 disables operation of the blade 22 by, for example, applying abrake to cease motion of the blade 22 and/or by moving the blade 22below the cutting area. One example of a reaction system 92 uses anexplosive charge to drive a brake (not shown) into the blade 22 toarrest the motion of the blade 22. In addition, or instead, anembodiment of the reaction system 92 collapses a blade support member(not show) to urge the blade 22 below the surface of the table 14.

The embodiment of the detection system 90 shown in FIG. 1 includes anoscillator 10 that generates a time-varying signal on line 12. Thetime-varying signal is any suitable signal type including, for example,a sine wave, a sum of multiple sine waves, a chirp waveform, a noisesignal, etc. The frequency of the signal is chosen to enable a detectionsystem to distinguish between contact with the first object, such as afinger or hand, and a second object, such as wood or other material, tobe cut by the power tool. In the embodiment of FIG. 1, the frequency is1.22 MHz, but other frequencies can also be used, as well asnon-sinusoidal wave shapes. The oscillator 10 is referenced to the sawtable 14 or other metallic structure as a local ground. As shown in FIG.1, the blade 22 is disposed vertically in an opening defined by the sawtable 14 (or work surface or cutting surface or platform).

The oscillator 10 is connected to two voltage amplifiers or buffers 16,18 through the line 12. The first voltage amplifier 16 has an outputconnected to line 20, which operatively connects the output of theoscillator to the saw blade 22. A current sensor 24 operatively connectsa signal from line 20 onto line 26 that is fed to an amplifier 28, whichis connected to a processor 30 by line 32. The current sensor 24 is, forexample, a current sense transformer, a current sense resistor, a HallEffect current sense device, or other suitable type of current sensor.An output line 34 from the processor 30 is operatively connected to thereaction system 92 so that the processor 30 triggers the reaction system92 if predetermined conditions are detected indicating, for example,contact between the blade 22 and the first object.

The signal on line 26 is indicative of the instantaneous current drawnby the blade 22. Because the saw blade 22 is in motion during operationof the table saw, the connection is made through an excitation plate 36,which is mounted generally parallel to the blade 22. The plate 36 isdriven by the first voltage amplifier 16, and is configured with acapacitance of approximately 100 picoFarad (pF) relative to the blade 22in the embodiment of FIG. 1. The plate 36 is held in a stable positionrelative to the side of the blade 22. The excitation plate 36 isconfigured to follow the blade 22 as the height and bevel angle of theblade 22 are adjusted during operation of the saw 1.

The capacitance between the first object and the saw table 14 (or powerline ground if one is present) is in the range of approximately 30-50 pFin the embodiment of FIG. 1. When the capacitance between the excitationplate 36 and the saw blade 22 exceeds the capacitance between the firstobject and the saw table 14, the detection thresholds are not undulyaffected by changes in the plate-to-blade capacitance. In theconfiguration of FIG. 1, the plate 36 is arranged in parallel with theblade 22 on the side where the blade 22 rests against the arbor 37, sothat changes in blade thickness do not affect the clearance between theblade 22 and the plate 36. Other methods of excitation, includingcontact through the arbor bearings or brush contact with the shaft orthe blade, could be used to the same effect.

In the detection system 90, the second-amplifier 18 is connected to ashield 38, and the amplifier 18 drives the shield 38 to the samepotential as the excitation plate 36. Also, sensors in the detectionsystem 90 optionally monitor the level of electrical current drawn bythe shield 38. The shield 38 extends around the blade 22 underneath thetable 14, and is spaced some distance away from the blade 22 on the topof the table 14 in the configuration of FIG. 1. The configuration of theshield 38 reduces the static capacitance between the blade 22 and thetable 14, which acts as a ground plane if the table is not electricallyconnected to an earth ground. In various embodiments, the shield 38 is acontinuous pocket of mesh, or some other type of guard that iselectrically equivalent to a Faraday cage at the excitation frequenciesgenerated by the oscillator 10. The shield 38 optionally includes acomponent that moves with the blade adjustments, or is large enough toaccommodate the blade's adjustment as well as the various blades thatfitted on the table saw. In the configuration of FIG. 1, the shield 38moves with the blade adjustments, and includes a throat plate area ofthe table top 14.

The processor 30 performs various pre-processing steps and implements atrigger that enables detection of conditions indicative of contactbetween the first object and the blade 22. The processor 30 optionallyincludes one or more associated analog-to-digital (A/D) converters. Theblade current signal from the current sensor 24 is directed to one ormore of the A/D converters, which generate a corresponding digitalsignal. A blade voltage signal representing the voltage differencebetween the blade 22 and the excitation plate 36 is directed an A/Dconverter to generate a digital blade voltage signal in someembodiments. The processor 30 receives the digitized signal and performsvarious digital signal processing operations and/or computes derivativeparameters based on the received signal. The processor 30 analyzes orotherwise performs operations on the conditioned blade signal to detectconditions indicative of contact between the first object and the blade22.

The prior art saw requires that the blade 22 be formed from anelectrically conductive material that is also electrically connected tothe arbor 37. Non-conductive blades and blades that includenon-conductive coatings prevent proper operation of the contactdetection system in the prior art saws. Additionally, the blade 22 andarbor 37 must be electrically connected to a ground plane for thecontact detection system to operate effectively. The requirement for aground connection to the blade also requires the saw 1 to beelectrically connected to a proper ground, such as a ground spike, metalpipe, or other suitable ground, which requires that the table saw 1remain in a fixed location. Other types of table saws include portabletable saws that are transported between job sites where providing aground connection may be inconvenient or impractical. Additionally, therequirement for a ground connection increases the complexity of setupand operation of non-portable table saws. Consequently, improvements tocontact detection systems that do not require an electrical groundconnection for the blade in portable and non-portable table saws wouldbe beneficial.

SUMMARY

In one embodiment, a detection system that detects contact between animplement in a saw and an object has been developed. The system includesan electrically conductive plate positioned at a predetermined distancefrom the implement, a detection circuit comprising a transformer, and asingle cable. The transformer includes a first winding formed from afirst electrical conductor between a first terminal and a secondterminal, a second winding formed from a second electrical conductorbetween a third terminal and a fourth terminal. The single cableconnects the first terminal and the second terminal of the winding tothe charge plate and the implement. The single cable includes a firstconductor electrically connected to the first terminal of the windingand to the electrically conductive plate, a second conductorelectrically connected to the implement, and an electrical insulatorpositioned between the first conductor and the second conductor.

In a further embodiment, the first conductor is a central conductor in acoaxial cable, the second conductor is a ribbon conductor in the coaxialcable surrounding the first conductor and the insulator is arrangedbetween the first conductor and the second conductor in the coaxialcable.

In a further embodiment, the detection circuit includes a firstdemodulator electrically connected to the third terminal of the secondwinding, a second demodulator electrically connected to the fourthterminal of the second winding, a clock generator electrically connectedto the first winding, the clock generator is configured to generate asensing signal through the first winding at a predetermined frequency,and a controller configured to receive an in-phase signal from the firstdemodulator and a quadrature-phase signal from the second demodulator.

In a further embodiment, the system includes an implement reactionmechanism operatively connected to the implement, and the controller isoperatively connected to the implement reaction mechanism. Thecontroller is further configured to identify a spike in the sensingsignal with reference to the in-phase signal from the first demodulatorand the quadrature-phase signal from the second demodulator, andgenerate a control signal to operate the implement reaction mechanism inresponse to identification of the spike.

In a further embodiment, the detection circuit includes a firstthyristor electrically connected between the third terminal of thesecond winding and the first demodulator, and a second thyristorelectrically connected between the fourth terminal of the second windingand the second demodulator.

In a further embodiment, the saw includes an implement enclosure and

an arbor connected to the implement enclosure and the implement, thesecond conductor is electrically connected to the implement through theimplement enclosure and the arbor.

In a further embodiment, the implement enclosure includes a heightadjustment carriage and a bevel carriage, and the second conductor iselectrically connected to the height adjustment carriage in a firstlocation and the bevel carriage in a second location.

In a further embodiment, system includes a metal sleeve mounted to theimplement enclosure and surrounding a portion of the second conductor toestablish the electrical connection between the second conductor and theimplement through the implement enclosure.

In a further embodiment, the first conductor is one conductor in asingle twisted pair cable, the second conductor is a second conductor inthe twisted pair cable, and the insulator separates the first conductorand the second conductor in the twisted pair cable.

In a further embodiment, the twisted pair cable includes a metallicshield surrounding the first conductor, the second conductor, and theinsulator.

In a further embodiment, the system includes a first printed circuitboard (PCB) supporting the first detection circuit, a second PCBsupporting a power supply and a TRIAC, a data cable operativelyconnected to the first PCB and the second PCB to enable the detectioncircuit to transmit a control signal from the first PCB to the secondPCB, and a ferrite choke formed around the data cable.

In a further embodiment, the system includes a tamp resistor positionedon the first PCB, the tamp resistor is connected to the data cable andan electrical ground on the first PCB.

In a further embodiment, the system includes a table including anopening for the implement and a first electrical cable connecting thetable to an electrical ground. The table is electrically isolated fromthe implement and the plate.

In a further embodiment, the system includes a second cable electricallyconnected to an implement enclosure and to the electrical ground througha first resistor and a third cable electrically connected to theimplement and to the electrical ground through a second resistor.

In a further embodiment, the first resistor and the second resistor eachhave a resistance level of approximately 1 MΩ.

In a further embodiment, the saw includes a rip fence positioned over asurface of the table, the rip fence including a first electricalinsulator positioned between the rip fence and the table and a secondelectrical insulator positioned above the surface of the rip fence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a prior art table saw including a prior artdetection system for detecting contact between a human and a saw blade.

FIG. 2 is a schematic diagram of a table saw including an objectdetection system configured to identify if a saw blade in the sawcontacts an object during rotation of the saw blade.

FIG. 3 is an external view of one embodiment of the table saw of FIG. 2.

FIG. 4 is a cross-sectional view of selected components including theblade, arbor, and sensor plate in the saw of FIG. 2.

FIG. 5A is an external view of a user interface device in the saw ofFIG. 2.

FIG. 5B is a view of the user interface device of FIG. 5A with anexternal housing removed.

FIG. 5C is a profile view of the user interface device of FIG. 5B.

FIG. 5D is an exploded view of components in the user interface of FIG.5A-FIG. 5C.

FIG. 6A is an exploded view of a charge coupled plate and arbor assemblyin one embodiment of the saw of FIG. 2.

FIG. 6B is a profile view of the components depicted in FIG. 6A.

FIG. 7 is a schematic diagram depicting additional details of the objectdetection system and other components in one embodiment of the saw ofFIG. 2.

FIG. 8A is a diagram depicting a sensing cable installed in oneembodiment of the saw of FIG. 2.

FIG. 8B is a cut away diagram of components in a coaxial sensing cable.

FIG. 8C is a diagram depicting a connection of a first conductor in thesensing cable to a plate in the saw of FIG. 8A.

FIG. 8D is a diagram depicting a mount at one location for connection ofa second conductor in the sensing cable to an implement enclosure in thesaw of FIG. 8A.

FIG. 8E is a diagram depicting a mount at another location forconnection of a second conductor in the sensing cable to an implementenclosure in the saw of FIG. 8A.

FIG. 9A is a schematic diagram of capacitive sensors arranged in athroat plate around a blade in one embodiment of the saw of FIG. 2.

FIG. 9B is a block diagram of a process for operation of a table sawusing the capacitive sensors of FIG. 9A.

FIG. 10 is a block diagram of a process for monitoring activity of theimplement reaction mechanism in one embodiment of the saw of FIG. 2 anddisabling the saw for maintenance after the number of activations of theimplement reaction mechanism exceeds a predetermined number.

FIG. 11 is a block diagram of a process for measuring profiles ofdifferent types of materials used in work pieces for the objectdetection system in the saw of FIG. 2.

FIG. 12 is a block diagram of a process for measuring the capacitance inthe body of an operator of the saw to adjust operation of the objectdetection system in the saw of FIG. 2.

FIG. 13A is a schematic view of components in the motor of oneembodiment of the saw of FIG. 2.

FIG. 13B is a block diagram of a process for measuring wear on a brushin the motor depicted in FIG. 13A based on electrical resistance in thebrush.

FIG. 13C is a block diagram of a process for measuring wear on a brushin the motor depicted in FIG. 13A based on a pressure measurement for aspring that biases the brush to a commutator in the motor.

FIG. 14 is a block diagram of a process for diagnosing faults in thesensing cable of one embodiment of the saw of FIG. 2.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theembodiments described herein, reference is now made to the drawings anddescriptions in the following written specification. No limitation tothe scope of the subject matter is intended by these references. Thispatent also encompasses any alterations and modifications to theillustrated embodiments as well as further applications of theprinciples of the described embodiments as would normally occur to oneskilled in the art to which this document pertains.

As used herein, the term “power tool” refers to any tool with one ormore moving parts that are moved by an actuator, such as an electricmotor, an internal combustion engine, a hydraulic or pneumatic cylinder,and the like. For example, power tools include, but are not limited to,bevel saws, miter saws, table saws, circular saws, reciprocating saws,jig saws, band saws, cold saws, cutters, impact drives, angler grinders,drills, jointers, nail drivers, sanders, trimmers, and routers. As usedherein, the term “implement” refers to a moving part of the power toolthat is at least partially exposed during operation of the power tool.Examples of implements in power tools include, but are not limited to,rotating and reciprocating saw blades, drill bits, routing bits,grinding disks, grinding wheels, and the like. As described below, asensing circuit integrated with a power tool is used to halt themovement of the implement to avoid contact between a human operator andthe implement while the implement is moving.

As used herein, the term “implement reaction mechanism” refers to adevice in a saw that retracts an implement, such as a blade or any othersuitable moving implement, from a location with potential contact with awork piece or a portion of the body of a human operator, that halts themotion of the implement in a rapid manner, or that both retracts andhalts the implement. As described below in a table saw embodiment, oneform of implement reaction mechanism includes a movable drop arm that ismechanically connected to an implement, such as a blade, and an arbor.The implement reaction mechanism includes a pyrotechnic charge that isoperated by an object detection system in response to detection ofcontact between a portion of the body of an operator and the bladeduring operation of the saw. The pyrotechnic charges force the drop armand blade below the surface of the table to retract the blade fromcontact with the operator in a rapid manner. In other embodiments of theimplement reaction mechanism, a mechanical or electromechanical bladebrake halts the movement of the blade in a rapid manner.

FIG. 2 depicts a schematic view of components in a saw 100, while FIG. 3depicts an external view of one embodiment of the saw 100. The table saw100 includes a table 104 through which a saw blade 108 extends forcutting work pieces, such as pieces of wood. The table saw 100 alsoincludes an electric motor 112 that rotates an arbor 109 to drive thesaw blade 108, an implement enclosure 118, and an implement reactionmechanism 132. While FIG. 2 depicts a cutting blade 108 for illustrativepurposes, those of skill in the art will recognize that the blade 108may be any implement that can be used in the saw 100 and that thereferences to the blade 108 are for illustrative purposes. In the saw100, the implement enclosure 118 includes a height adjustment carriageand a bevel carriage that surround the blade 108, and the implementenclosure 118 is alternatively referred to as a blade enclosure or“shield” that surrounds the blade 108 or other suitable implement in thesaw 100. As depicted in FIG. 3, a portion of the blade 108 extendsupward through an opening in the throat plate 119 above the surface ofthe table 104. A riving knife 330 and blade guard 332 are positionedover the blade 108.

Within the saw 100, the implement enclosure 118 is electrically isolatedfrom the blade 108, arbor 109, the top surface of the table 104, and aplate 120. In one embodiment, the implement enclosure 118 includes athroat plate 119 that is formed from an electrical insulator, such asthermoplastic. The throat plate 119 includes an opening to enable theblade 108 to extend above the surface of the table 104. The throat plate119 is level with the surface of the table 104 and provides furtherelectrical isolation of the blade 108, height adjustment carriage, andbevel carriage in the implement enclosure 118 from the surface of thetable 104. The general configuration of the table 104, blade 108, andmotor 112 are well known to the art for use in cutting work pieces andare not described in greater detail herein. Some components that arecommonly used in table saws, such as guides for work pieces, bladeheight adjustment mechanisms, and blade guards are omitted from FIG. 2for clarity.

The saw 100 further includes an object detection system 102 that isincludes a digital controller 140, memory 142, clock source 144,amplifier 146, transformer 150 and demodulators 143A and 143B. Theobject detection system 102 is electrically connected to the plate 120and to the blade 108 via the implement enclosure 118 and arbor. Thecontroller 140 in the object detection system 102 is operativelyconnected to the user interface device 110, motor 112, and implementreaction mechanism 132. During operation of the saw 100, the bladedetection system 102 detects electrical signals that result from changesin the capacitance levels between the blade 108 and the plate 120 whenan object contacts the rotating blade 108. An object can include a workpiece, such as a piece of wood or other material that the saw 100 cutsduring ordinary operation. The object detection system 102 also detectscontact between the blade 102 and other objects, including potentially ahand or other portion of the body of the operator of the saw, andactivates the implement reaction mechanism 132 in response to detectionof contact between the blade 108 and objects other than work pieces.Additional structural and operational details of the object detectionsystem 102 are described in more detail below.

In the saw 100, the table 104 is electrically isolated from the sawblade 108, arbor 109, and other components in the saw enclosure 118 asdepicted in FIG. 2 and FIG. 3. In one embodiment, the surface of thetable 104 is formed from an electrically conductive metal, such as steelor aluminum. At the surface of the table 104, the electricallynon-conductive throat plate 119 isolates the blade 108 from the surfaceof the table 104. Under the table 104, one or more electricallyinsulated mounts that secure the table 104 to the frame of the saw 100but electrically isolate the table 104 from other components within thesaw. As depicted in FIG. 2, in some embodiments the table 104 iselectrically connected to ground 182 with an electrical cable. Theground connection reduces or eliminates the buildup of staticelectricity on the table 104, which prevents errant static dischargesthat can reduce the accuracy of object detection during operation of thesaw 100.

In addition to the ground connection for the table 104, the blade 108and implement enclosure 118 are connected to the ground 182 through highresistance cables that incorporate large resistors 180 (e.g. 1 MΩresistors). The implement enclosure 118 is connected to ground 182through a first cable and a resistor 180 that provides a high-resistanceconnection to ground. The blade 108 is also connected to the ground 182via the arbor 109 through a second cable and resistor 180. Thehigh-resistance connections to ground for the blade 108 and implementenclosure 118 also reduce the buildup of static charge on thesecomponents. While prior art detection devices require a low-resistanceground connection (e.g. a direct connection using an electrical cablewith a resistance of less than 1Ω) in order to detect contact between ablade and an object using a low-impedance connection directly to earthground, the high-resistance ground cables in the saw 100 are notrequired for operation of the object detection system 102. Instead thehigh-resistance cables merely reduce the effects of static electricityin the saw 100 to reduce potential false-positive detection events, butthe object detection system 102 is still fully functional to detectcontact between the blade 108 and an object without any groundconnection. Alternative embodiments use different materials for eitheror both of the plate 120 and blade 108 to reduce the buildup of staticelectricity in the saw 100 and do not require any connection between theblade 108 or implement enclosure 118 and ground.

The table saw 100 includes a rip fence 304 that is mounted on rails 310and 312. The rip fence 304 is configured to move to a predeterminedposition over the table 304 with an orientation that is parallel to theblade 108 to guide work pieces through the saw 100 during operation. Inthe saw 100, the rip fence 304 is electrically isolated from the table104. For example, in FIG. 3 an electrically insulated thermoplastic railmount 306 couples the rip fence 304 to the rail 310. A plastic guard(not shown) on the bottom of the rip fence 304 and another guard 320 onthe top of the rip fence 304 electrically isolate the rip fence 304 fromthe table 104 in the saw 100. In some embodiments, the rip fence 304includes another electrical insulator positioned on the side of the ripfence 304 that faces the blade 108 to ensure electrical isolationbetween the rip fence 304 and the blade 108 when a work piece engagesboth the rip fence 304 and blade 108 simultaneously.

Referring again to FIG. 2, the saw 100 also includes the detectionsystem 102 that detects contact between objects and the blade 108 duringoperation of the saw 100. In one configuration, some or all of thecomponents in the detection system 102 are mounted to one or moreprinted circuit boards (PCBs). In the embodiment of FIG. 2, a separatePCB 172 supports a power supply 106 and a control TRIAC 174. The powersupply 106 receives an alternating current (AC) electrical power signalfrom an external power source, such as a generator or electrical utilityprovider, and supplies electrical power to the motor 112 through theTRIAC 174 and to supply electrical power to the components in thesensing system 102. The separate PCBs for the sensing system 102 andpower supply 172 isolate the digital controller 140 from the powersupply 106 and TRIAC 174 to improve cooling of the digital electronicsin the controller 140 and to isolate the controller 140 from electricalnoise. In the embodiment of FIG. 2, the power supply 106 is a switchedpower supply that converts the AC power signal from an external powersource to a direct current (DC) electrical power signal at one or morevoltage levels to supply power to the controller 140, clock source 144,and amplifier 146. The detection system 102 and the components mountedon the detection system 102 are electrically isolated from an earthground. The power supply 106 serves as a local ground for the componentsmounted to the detection system 102.

In the saw 100, the plate 120 and the blade 108 form a capacitor 124where a small air gap between the plate 120 and the blade 108 acts as adielectric. The plate 120 is an electrically conductive plate such as asteel or aluminum plate that is positioned at a predetermined distancefrom the blade 108 with a parallel orientation between the plate 120 andthe blade 108 to form two sides of the capacitor 124 with an air gapdielectric. The transformer 150 includes a first winding 152 and asecond winding 154. In the saw 100, the plate 120 is a metallic planarmember that is electrically connected to the winding 152 in thetransformer 150. The plate 120 is otherwise electrically isolated fromthe implement enclosure 118 and is electrically isolated from the blade108 by a predetermined air gap to form the capacitor 124. The plate 120is also referred to as a charge coupled plate (CCP) because the plate120 forms one side of the capacitor 124 in conjunction with the blade108. In one embodiment, a plastic support member holds the plate 120 ina predetermined position with respect to the blade 108. The blade 108and blade arbor 109 are electrically isolated from the enclosure 118,plate 120, the drop arm in the implement reaction mechanism 132, andother components in the saw 100. For example, in the saw 100, one ormore electrically insulated plastic bushings isolate the arbor 109 andblade 108 from the implement enclosure 118, the drop arm in theimplement reaction mechanism 132, and other components in the saw 100.Additionally, the saw blade 108 and arbor 109 are electrically isolatedfrom ground. Thus, the blade object detection system in the saw 100operates in an “open loop” configuration where the capacitor 124 isformed from the plate 120 and the blade 108 while the blade 108 andarbor 109 remain electrically isolated from the other components in thesaw 100. The open loop configuration increases the capacitance betweenthe plate 120 and the saw blade 108 in comparison to the prior artsensing systems where the saw blade is electrically grounded. The largercapacitance in the saw 100 improves the signal to noise ratio fordetection of a signal that indicates contact between a human operationand the saw blade 108.

As depicted in FIG. 2, the plate 120 is electrically connected to oneside of the first winding 152 in the transformer 150 while the implementenclosure 118 is electrically connected to the other side of the firstwinding 152. In one embodiment, the saw 100 includes a single coaxialcable that includes two electrical conductors to establish the twoelectrical connections. In one configuration, the center conductorelement of the coaxial cable is connected to the plate 120 and the firstterminal of the first winding 152 in the transformer 150. The outersheath of the coaxial cable is electrically connected to the blade 108through the enclosure 118 and the arbor 109 and to the second terminalof the first winding in the transformer 150. The structure of thecoaxial cable provides shielding to transmit the electrical signals fromthe plate 120 and implement enclosure 118 while attenuating electricalnoise that is present in the saw 100.

FIG. 4 depicts a cross-sectional view of the blade 108, arbor 109, andplate 120 in more detail. In FIG. 4, electrically nonconductive bushings404 and 408 engage the arbor 109. The electrically nonconductivebushings 404 and 408 include, for example, layers of electricallyinsulated plastic, ceramic, or other insulators that electricallyisolate the arbor 109 from other components in the saw 100. In theillustrative example of FIG. 4, the bushings 404 and 408 includebearings that enable the arbor 109 to rotate during operation. The blade108 only physically engages the arbor 109, and remains electricallyisolated from other components in the saw 100. In FIG. 4, a plasticsupport member 412 holds the plate 120 in position at a predetermineddistance from the blade 108 while electrically isolating the plate 120from other components in the saw 100.

FIG. 6A and FIG. 6B depict an exploded and front view, respectively, ofthe components depicted in FIG. 4. FIG. 6A depicts the plate 120 and thesupport member 412, which are affixed to the support frame that holdsthe arbor 109 using a set of screws. To maintain electrical isolationbetween the plate 120 and the arbor 109 and other components in theenclosure 118, the screws are either electrically non-conductive or thethreaded holes in the support frame include electrically non-conductivethreadings to maintain the electrical isolation. The support member 412includes a lip 612 that surrounds the outer perimeter of the plate 120and extends outward past the surface of the plate 120. The lip 612provides additional protection and electrical isolation to the plate 120during operation of the saw 100. In particular, the lip 612 preventscontact between the blade 108 and the plate 120 due to potentialtransient wobbles in the rotation of the blade 108 as the blade 108 cutswork pieces during operation of the saw 100. FIG. 6B further depicts thelip 612 of the support member 412 that extends around the plate 120.

FIG. 7 depicts additional details of one embodiment of the objectdetection system 102 and power supply and control PCB 172 of FIG. 2 inmore detail. In the configuration of FIG. 7, some of the cablesconnecting different components in the saw 100 include ferrite chokes,such as ferrite chokes 708, 738, and 740 that are coupled to cables 724,736, and 742, respectively. The cable 742 connects the TRIAC 174 to themotor 112 and the ferrite choke 740 reduces noise in the electricalcurrent that passes through the cable 742 to supply power to the motor112 upon activation of the TRIAC 174. As discussed in more detail below,the ferrite chokes 708 and 738 reduce noise in the data and power cables724 and 736, respectively, which connect the object detection system 102to the power supply and control PCB 172. In the configuration of FIG. 7,the sensing cable 720 that includes the first conductor connected to theplate 120 and the second conductor electrically connected to the sawblade 108 does not pass through a ferrite choke. Similarly, a motortachometer cable (not shown) connecting the motor 112 to the controller140 does not pass through a ferrite choke. As is known in the art, theferrite chokes filter high-frequency noise from the cables that areconnected to the controller 140 and other components in the objectdetection system.

FIG. 7 also depicts thyristors 743A and 743B. The thyristor 743Aconnects the third terminal of the transformer 150 to the demodulator143A for demodulation of the in-phase component of the sensing signal.The thyristor 743B connects the fourth terminal of the transformer 150to the second demodulator 143B for the quadrature phase component of thesensing signal. The thyristors 743A and 734B are “two lead” thyristors,which are also referred to as Shockley diodes, that switch on inresponse to an input signal that exceeds a predetermined breakdownvoltage but do not require a separate gate control signal to be placedin the switched on state. The thyristors 743A and 743B are configuredwith a breakdown voltage that is somewhat higher than the normal voltageamplitude of sensing signal to reduce the effects of random noise in theinputs of the demodulators 143A and 143B. However, if an object such ahuman hand contacts the blade 108, then the input voltages exceed thebreakdown threshold level of the thyristors 743A and 743B and both thethyristors 743A and 743B switch on to enable the spike and the sensingsignal to pass to the demodulators 143A and 143B, respectively. Thethyristors 743A and 743B are optional components in the embodiment ofFIG. 7 and alternative configurations of the object detection system 102omit these thyristors.

In FIG. 7, the data cable 724 that connects the controller 140 to thepower supply 106 and TRIAC 174 on the power supply PCB 172 passesthrough the ferrite choke 708. Additionally, a pull-down resistor 732connects the data cable 724 between the controller 140 and the powersupply PCB 172 to a local ground (e.g. a copper ground plane on the PCBof the object detection system 102) to provide additional noisereduction in signals that are transmitted over the cable 724. Thepull-down resistor and ferrite choke enable the data cable 724 to carrycontrol signals using a predetermined command protocol, such as I²C,over a long distance between the first PCB of the object detectionsystem 102 and the second PCB 172 of the power supply 106 and TRIAC 174.For example, in one configuration of the saw 100, the data cable 724 hasa length of approximately 0.75 meters and transmits the I²C signals fromthe controller 140 to the power supply 108 and command logic associatedwith the TRIAC 174. The power cable 736 that provides electrical powerfrom the power supply 106 to the controller 140 and other components inthe object detection system 102 passes through the ferrite choke 738.While FIG. 7 depicts a separate data cable 724 and power cable 736, inanother embodiment a single cable provides both power and dataconnectivity between power supply PCB 172 and the components in theobject detection system 102. The single cable embodiment also uses aferrite choke to reduce the effects of noise in a similar manner to theconfiguration of FIG. 7.

FIG. 8A-FIG. 8E depict the coaxial cable that connects the plate 120 andblade 108 to the detection system 102 in more detail. FIG. 8A depicts anenclosure 802 that contains the PCB and other components in the SCU thatimplements the object detection system 102 and other control elements ofthe saw 100. The sensing cable 720 is electrically connected to both thesensing plate 120 and the blade 108. As depicted in FIG. 8A and FIG. 8Bthe sensing cable 720 is a coaxial cable with a first internal conductor852, an electrical insulator 856 that surrounds the inner conductor 852and separates the inner conductors from a second metallic conductor 862,and an exterior insulator 864 surrounding the second conductor 862. Inthe configuration of FIG. 8A, the first conductor 852 is connected tothe plate 120 and to the first terminal of the transformer 150 in theobject detection system 102 as depicted in FIG. 2. The second conductor862 is electrically connected to the blade 108 and to the secondterminal of the transformer 150 in the object detection system 102 asdepicted in FIG. 2.

While FIG. 8B depicts a coaxial cable, an alternative embodiment employsa twisted pair cable that includes two different conductors that aretwisted around one another in a helical pattern. One or both of theconductors in the twisted pair cable are surrounded by an electricalinsulator to isolate the conductors from each other. Additionally, ashielded twisted pair cable includes an external shield, such as ametallic foil, that is wrapped around the twisted pair cable and reducesthe effects of external electrical noise on the conductors in thetwisted pair cable.

FIG. 8A depicts the connection of the single sensing cable 720 to theplate 120 at location 832 and to the bevel carriage and heightadjustment carriage of the implement enclosure 118 at locations 836 and838. FIG. 8C depicts the connection of the first conductor in thesensing cable 720 to the plate 120 at location 832 in more detail. Ametal retention clip 866 is affixed to the plate 120 and to the firstconductor 852 in the sensing cable 720 to establish the electricalconnection. In the configuration of FIG. 8C, the retention clip 866 isinserted between the plate 120 and the support member 412 to ensure astable connection between the sensing cable 720 and the plate 120. Insome embodiments, the retention clip 866 is soldered to the plate 120.

The second conductor 862 is electrically connected to the blade 108, butsince the blade 108 rotates during operation of the saw and since theblade 108 is typically a removable component, the second conductor 862is not physically connected to the blade 108 directly. Instead, thesecond conductor is connected to the implement enclosure 118. In somesaw embodiments, the enclosure 118 actually includes multiplecomponents, such as the height adjustment carriage and bevel carriage inthe saw 100. To ensure a consistent electrical connection, the secondconductor in the single sensing cable 720 is connected to each of theheight adjustment carriage and the bevel carriage to maintain a reliableelectrical connection with the blade 108. For example, in FIG. 8 thesecond conductor in the sensing cable 720 is connected to the heightadjustment carriage at location 836 and to the bevel carriage atlocation 838.

FIG. 8D and FIG. 8E depict two different mount locations that connectthe second conductor in the sensing cable 720 to the implement enclosure118 at two different locations including both the height adjustmentcarriage and bevel carriage. As depicted in FIG. 8D, the secondconductor is electrically and physically connected to the implementenclosure 118 at location 836 using a connection mount 872. Theoutermost insulator 864 is removed from the sensing cable 720 within theconnection mount 872 to establish an electrical connection with theimplement enclosure 118. In some embodiments, the connection mount 872is formed from a metal sleeve that surrounds and engages a portion ofthe second conductor 862 in the sensing cable 720. As described above,the implement enclosure 118 is electrically connected to the arbor 109and the blade 108, and the cable mount 872 provides a reliableelectrical connection between the second conductor 862 in the sensingcable 720 and the blade 108 through the height adjustment carriage. FIG.8E depicts another configuration of a connection mount 876 that securesthe sensing cable 720 at location 838 to the bevel carriage and providesan electrical connection between the second conductor 862 in the sensingcable 720 and the implement enclosure 118. In one embodiment, theconnection mount 876 is also formed from a metal sleeve that surrounds aportion of the second conductor in the sensing cable 720 to establishthe electrical connection with the blade 108 through the implementenclosure 118.

As depicted in FIG. 2 and FIG. 7, the controller 140 is operativelyconnected to the power supply 106 and TRIAC 174 on the separate PCB 172through a data line. In the embodiment of the saw 100, the data line isa multi-conductor cable such as an HDMI cable and the controller 140transmits command messages to the PCB 172 using the I²C protocol. Thecontroller 140 optionally receives status data or data from sensors,such as onboard temperature sensors, from the PCB 172 using the I²Cprotocol. The ferrite choke 708 reduces electrical noise in the datacable 724 and the ferrite choke 738 reduces electrical noise in thepower cable 736. The tamp resistor 732 also reduces noise through thedata cable 724. In one embodiment, the data cable 724 includes aphysical configuration that conforms to the High-Definition MultimediaInterface (HDMI) standard, which includes multiple sets of shieldedtwisted-pair conductors, although the data cable 724 does not transmitvideo and audio data during operation of the saw 100. In the embodimentof FIG. 2, the data cable has a length of approximately 0.75 meters toconnect the separate PCBs 102 and 172.

During operation, the controller 140 signals the TRIAC 174 to supplyelectrical current to the motor 112 through a gate in the TRIAC. Oncetriggered, the TRIAC 174 remains activated for as long as at least apredetermined level of electrical current from the power supply 106passes through the TRIAC 174 to power the motor 112. The power supply106 varies the amplitude of the current that is delivered to the motor112 to adjust the rotational speed of the motor 112 and saw blade 108.To deactivate the motor 112, the power supply reduces the level of powersupplied to the TRIAC 174 below a predetermined holding currentthreshold and the TRIAC 174 switches off. In the embodiment of FIG. 2,the TRIAC 174 enables operation of the motor 112 at varying speed levelsand activation/deactivation without requiring relays that are typicallyneeded in prior art power saws. In the illustrative example of FIG. 2,the TRIAC 174 passes an AC electrical signal to the motor 112, althoughalternative embodiments include DC motors that receive DC electricalpower instead.

The controller 140 and associated components in the detection system 102are sometimes referred to as a saw control unit (SCU). The SCU iselectrically isolated from other components in the saw 100 with theexception of the power, control, and sensor data connections between thedetection system 102 and other components in the saw 100. In the saw100, the controller 140 also handles control of other operations in thesaw 100 that are not directly related to the detection of object contactwith the blade 108, such as activating and deactivating the motor 112.In the embodiment of FIG. 2, the SCU is located outside of the implementenclosure 118, the detection system 102 is mounted to a non-conductiveplastic support member, and the detection system 102 is oriented toavoid placing the ground plane of the detection system 102 in parallelwith any metallic members within the saw 100 to reduce the transfer ofelectrical noise to the electrically conductive traces in the detectionsystem 102.

In the saw 100, the clock source 144 and driving amplifier 146 in thesensing circuit generate a time varying electrical signal that isdirected through a first winding 152 in the transformer 150, thecapacitive coupling plate 120, the blade 108, and the implementenclosure 118. The time varying electrical signal is referred to a“sensing current” because the controller 140 senses contact between theblade 108 and a portion of a human body with reference to changes in theamplitude of the sensing current. The time varying electrical signal isa complex valued signal that includes both an in-phase component andquadrature component. The sensing current passes through the firstwinding 152 in the transformer 150 to the plate 120. The changes in thefirst winding caused by discharges between the plate 120 and the blade108 produce an excitation signal in the second winding 154 of thetransformer 150. The excitation signal is another complex valued signalthat corresponds to the sensing current passing through the firstwinding 152.

The controller 140 in the sensing circuit is operatively connected tothe motor 112, the second winding 154 in the transformer 150, amechanical implement reaction mechanism 132. The controller 140 includesone or more digital logic devices including general purpose centralprocessing units (CPUs), microcontrollers, digital signal processors(DSPs), analog to digital converters (ADCs), field programmable gatearrays (FPGAs), application specific integrated circuits (ASICs) and anyother digital or analog devices that are suitable for operation of thesaw 100. The controller 140 includes a memory 142 that stores programmedinstructions for the operation of the controller 140, and datacorresponding to a threshold of max-min variations, a variancethreshold, or a frequency response threshold that are used to identifyif samples obtained from a sensing current flowing through the blade 108indicate that the saw blade 108 is rotating or is halted.

During operation of the sensing circuit, the clock source 144 generatesa time varying signal, such as sinusoidal waveform, at a predeterminedfrequency. In the embodiment of FIG. 2, the clock source 144 isconfigured to generate a signal at a frequency of 1.22 MHz, which isknown to propagate through the human body. The amplifier 146 generatesthe sensing current as an amplified version of the signal from the clocksource 144 with sufficient amplitude to drive the transformer 150 andcapacitor 124 for detection by the controller 140. In the embodiment ofFIG. 2, the saw 100 generates the sensing signal using amplitudemodulation (AM), but in alternative embodiments the sensing signal isgenerated with a frequency modulation, phase modulation, or othersuitable modulation technique.

During operation of the sensing circuit, the controller 140 receives thein-phase component I of the excitation signal in the second winding 154through a first demodulator 143A and the quadrature component Q of theexcitation signal through a second demodulator 143B. The transformer 150isolates the sensing current flowing through the first winding 152,plate 120, saw blade 108, and implement enclosure 118 from demodulators143A and 143B that supply the in-phase and quadrature phase componentsof the signal, respectively, to the controller 140. Since thedemodulators 143A and 143B generate electrical noise, the transformer150 reduces or eliminates the effects of the noise on the first winding152 and sensing current. In one configuration, the transformer 150 is a1:1 transformer where the first winding 152 and second winding 154 havean equal number of turns. In alternative configurations, the ratio ofwindings in the first winding 152 and second winding 154 are selected toeither step-up or step-down the signal for demodulation and monitoringby the controller 140. The controller 140 includes one or more ADCs,filters, and other signal processing devices required to generatedigital representations of the amplitude of the in-phase signal I andquadrature signal Q. The controller 140 identifies an amplitude of thesensing current A at a given time as a Pythagorean sum of the in-phaseand quadrature components in each sample, as illustrated in thefollowing equation: A=√{square root over (I²+Q²)}. The controller 140measures the demodulated signal at a predetermined frequency, such as a100 KHz sampling rate with a 10 μpec period between each sample, toidentify changes in the amplitude A of the complex valued signal.

As the motor 112 rotates the blade 108, the rotating blade 108 comesinto contact with different objects, including blocks of wood and otherwork pieces. A small portion of the charge that accumulates on the blade108 flows into the work piece. The electrical conductivity of the woodwork piece is, however, quite low, and the controller 140 in the sensingcircuit continues to enable the motor 112 to rotate the saw blade 108.For example, when the blade 108 engages a block of wood, the controller140 typically measures a small change in the sensing current A, but thechange in the sensing current is identified as corresponding to wood oranother material with low electrical conductivity.

While work pieces, such as wood, have low electrical conductivity,another object, such as a part of the human body, has a much higherelectrical conductivity and absorbs a much greater portion of the chargeon the blade 108 as the part approaches the blade 108. In FIG. 2 aportion of a human body 164, such as a hand, finger, or arm, isrepresented by a charge cloud indicating the flow of charge from theblade 108 to the human body. The contact between the human body and theblade 108 effectively changes the capacitance level, since the humanbody and saw blade 108 both receive charge from the sensing current. Thecontroller 140 identifies contact between the human body 164 and theblade 108 as a rapid increase in the amplitude A of the sensing currentat the time when the human body 164 contacts the blade 108. In responseto the rapid increase in the amplitude of the sensing signal, thecontroller 140 deactivates the motor 112, engages the implement reactionmechanism 132 to halt the motion of the blade 108, and optionallyretracts the blade 108 before the blade contacts the human body 164.

In the configuration of FIG. 2, the human body has sufficientconductivity and capacity to draw charge from the blade 108 even whenthe detection system 102 is isolated from earth ground and when thehuman body 164 is isolated from earth ground, such as when a humanoperator wears shoes with rubber soles. Thus, while the detection system102 and the human 164 do not share a common electrical ground, thecontroller 140 continues to identify contact between the human 164 andthe blade 108 through identification of a rapid increase in theidentified sensing current amplitude A. While the absolute value of theamplitude A may vary during operation of the saw 100, the controller 140can still identify contact with the human 164 in response to theamplitude and time of the increase in the relative value of theamplitude A. During operation of the saw 100, the controller 140 isconfigured to identify contact with the human 164 and to deactivate themotor 112 and engage the implement reaction mechanism 132 to halt thesaw blade 108 in a time period of approximately 1 millisecond.

In the saw 100, the controller 140 deactivates the electrical motor 112in response to identification of contact between the blade 108 and aportion of a human. In the saw 100, the saw blade 108 generallycontinues rotating for a period of several seconds due to the momentumthat the saw blade 108 accumulates during operation. The implementreaction mechanism 132 is configured to either halt the saw blade 108 ina much shorter period of time, to drop the saw blade 108 below the table104, which retracts the saw blade 108 from contact with the human, or toboth halt and retract the blade 108. In the saw 100, the implementreaction mechanism 132 includes a drop arm that is mechanicallyconnected to the saw blade 108. The implement reaction mechanism 132also includes a pyrotechnic charge that is configured to push the droparm down into the housing of the saw and away from the surface of thetable 104. The controller 140 operates the pyrotechnic charge to movethe drop arm and blade 108 downward in response to detection of contactbetween a portion of the body of the operator and the blade 108. Theimplement reaction mechanism retracts the blade 108 below the surface ofthe table 104.

In some configurations of the saw 100, the controller 140 is configuredto lock out operation of the saw 100 after the pyrotechnic device isfired a predetermined number of times. For example, in the configurationof the saw 100 the implement reaction mechanism 132 includes adual-pyrotechnic charge with a total of two “shots”. Each operation ofthe implement reaction mechanism consumes one pyrotechnic charge a in a“monoshot” operation. The operator removes and re-inserts thepyrotechnic device to place the second pyrotechnic charge in position tomove the drop arm in a subsequent operation of the implement reactionmechanism 132. The controller 140 stores a record of the number ofactivations of the implement reaction mechanism 132 and prevents the saw100 from being activated in a lockout process after the number ofactivations exceeds a predetermined number, such as one, two, or alarger number of activations. The controller 140 optionally sends anetwork notification to a service or warranty provider in embodiments ofthe saw 100 that are connected to a data network, such as the Internet,in the lockout operation. The lockout process enables service providersto diagnose potential issues with the operation of the saw 100 orprocedures for use of the saw 100 in response to operation of theimplement reaction mechanism 132 on a frequent basis.

In addition to sensing contact between an object and the saw blade 108when the saw blade 108 is moving, the sensing circuit in the saw 100 isconfigured to identify if the saw blade 108 is moving when the motor 112is deactivated. For example, the controller 140 identifies a period oftime when the saw blade 108 continues to rotate after an operatoroperates the user interface 110 to activate the saw 100 to cut one ormore work pieces, and subsequently opens the operates the user interface110 to deactivate the motor 112. The user interface 110 includes, forexample, an activation/deactivation switch to operate the saw 100, aspeed control input device, and status indicator lights that provideinformation about the operational status of the saw 100, such as if thesaw is ready for operation or has developed a fault. The user interfacedevice 110 is also referred to as a human machine interface (HMI).

The saw 100 is configured to be operated with the blade 108 and bladearbor 109 being isolated from electrical ground. The control electronicson the boards 102 and 172, the plate 120 and the implement enclosure 118may not be connected to a true earth ground in some configurations, butthese components share a common ground plane formed by, for example, ametal chassis of the saw or a ground plane formed on the circuit boardsof 102 and 172. As described above, during the contact detection processthe controller 140 identifies a spike in the current level for thesensing signal. However, electrical noise that is generated within thesaw 100 could produce false positive or false negative detection eventssince noise interferes with detection of the sensing signal. In the saw100, the PCBs 102 and 172 include ferrite core chokes that act aslow-pass filters to reduce the effects of noise. Additionally, the powercabling and data cabling pass through ferrite cores to reduce the noise.The power supply 106 includes a ferrite choke and a thyristor to rejectlow-speed transient noise in the electrical power signal that isreceived from the electrical power grid, a generator, or otherelectrical power source.

FIG. 5A-FIG. 5D depict a portion of one embodiment of the user interfacedevice 110 in more detail. FIG. 5A depicts an exterior view of a devicestatus display including an external housing 502, indicator lights528A-528D, and a covering for a short-range antenna 508. Duringoperation, the controller 140 activates one or more of the lights528A-528D to indicate different status information about the saw 100.For example, the light 528A indicates that the saw 100 is ready foroperation. The light 528B indicates that the implement reactionmechanism 132 has operated and that the pyrotechnic charge in theimplement reaction mechanism 132 should be reset. The light 528Cindicates that the user should look up a fault code. The light 528Dindicates that the saw 100 requires maintenance to replace a componentin the saw, such as a motor brush, or that the saw 100 requiresmaintenance after the implement reaction mechanism has operated for morethan a predetermined number of times. As depicted in FIG. 5A, theindicator lights 528A-528D provide a simplified interface. Alternativeembodiments include a different arrangement of indicator lights orinclude additional input and output devices including, for example,video display screens, touch input devices, and the like.

While the display indicator lights 528A-528D provide simplified directoutput feedback to the operator for regular use with the saw 100, insome circumstances the saw 100 transmits more complex diagnostic andconfiguration data to external devices. The controller 140 and userinterface device 110 optionally transmit more complex diagnostic dataand other information about the saw 100 to an external computing devicevia the short-range wireless antenna under the cover 512. Examples ofdiagnostic data that the controller 140 collects and optionallytransmits with the wireless transceiver and antenna 516 include,presence of voltage in the sensing circuit, the level of the sensorsignal, status information to indicate if the pyrotechnic device (pyro)in the implement reaction mechanism 132 is armed or disarmed, generate atest signal for the pyro firing line without sending a signal withsufficient amplitude to trigger monoshot operation of the pyro, detectpresence or absence of the pyro, check a resistance range for corrosionor wire damage in the sensor cable connected to the plate 120 andimplement enclosure 118 or other cables in the saw 100, generate a“tackle pulse” to identify a wire break in the line that provides powerto the motor 112, and identify faults in the motor 112 during a power onself-test.

As depicted in FIG. 5B, the short-range wireless antenna 516 is formedfrom a predetermined arrangement of conductive traces on a PCB thatsupports the indicator lights 528A-528D. FIG. 5B and FIG. 5C depictoptically translucent caps 504A-504D that form the exterior visiblesurface of each of the lights 528A-528B, respectively. The externalhousing 502 protects the antenna 516 from external elements whileenabling the antenna to be located on the exterior of the saw 100 tocommunicate with external electronic devices. The antenna 516 isoperatively connected to a wireless transceiver, such as an NFC,Bluetooth, IEEE 802.11 protocol family compatible (“Wi-Fi”), or othersuitable short-range wireless transceiver. An external electronicdevice, such as a smartphone, tablet, portable notebook computer, orother mobile electronic device receives data from the saw via a wirelesscommunication channel and optionally transmits information to the saw100 using the wireless communication channel. For example, a smartphonereceives diagnostic data from the saw 100 and a software applicationthat is run on the smartphone displays detailed diagnostic informationto an operator or maintenance technician to assist in maintenance of thesaw 100. The software application optionally enables the operator toinput configuration information for operational parameters of the saw100 that are not directly accessible through the simplified input device110. For example, in one configuration the software application enablesthe operator to input a maximum RPM rate for the motor 112 and blade108. In another configuration, the software application enables theoperator to transmit an identifier for a type of material that the saw100 will cut during operation, such as different types of wood,ceramics, plastics, and the like.

In another configuration, the saw 100 includes a lockout mechanism toprevent operation of the saw 100 unless a mobile electronic device withan appropriate cryptographic key is within a predetermined distance ofthe saw 100. The mobile electronic device transmits an encryptedauthorization code to the saw 100 in response to a query from the saw100 to unlock the saw 100 for operation. When the mobile electronicdevice is removed from proximity from the saw 100, a subsequent queryfails and the saw 100 remains inactive.

FIG. 5C depicts a profile view of the indicator lights 528A-528D. Eachlight includes an optically translucent cap, such as the cap 504A on thelight 528A, and an optically opaque body member 524A directs light froma light source, such as an LED, to the translucent cap. In the indicatorlight 528A, an LED 552 that is mounted on the PCB projects light throughan opening in the opaque body member 524A and the translucent cap 504A.The opaque body member 524A has a tapered shape with a narrow endsurrounding a first opening for the LED 552A and a wider end with asecond opening that engages the translucent cap 504A. The opticallyopaque member 524A prevents the light from the LED 552A from bleedingand producing false illumination in any of the other indicator lights528B-528D. The configuration of FIG. 5C enables the indicator lights inthe user interface device 110 to operate in direct daylight conditionsand prevents false illumination of incorrect indicator lights duringoperation.

FIG. 5D depicts an exploded view of select components from FIG. 5A-FIG.5C. FIG. 5D depicts the indicator cap assembly 540, which is formed froma molded plastic member that includes the translucent indicator lightcaps 504A-504D for the lights 528A-528D. The indicator cap assembly 540also includes an attachment member, such as the hook 506 that is formedfrom the molded plastic member of the indicator cap assembly 540, tosecure the caps to other components in the user interface device 110.The body member assembly 544 is another molded plastic member thatincludes the optically opaque body members 524A-524D corresponding tothe caps 504A-504D. Each of the optically opaque body members 524A-524Dincludes a first opening that aligns with one of the LEDs 552A-552D anda second opening that engages one of the caps 504A-504D. The body memberassembly 544 also includes attachment members, such as the hooks 526,which connect the opaque body members to other components in the userinterface device 110. The PCB 550 includes physical mounting locationsand electrical connections for the operation of the user interfacedevice 110. In particular, FIG. 5D depicts light emitting diodes (LEDs)552A-552D that are aligned with first openings in the correspondingopaque members 524A-524D and that provide light for the caps 504A-504Dof the indicator lights 528A-528D. The PCB 550 also includes the antenna516, which is formed from a predetermined pattern of conductive traceson the PCB, to enable wireless communication with the user interfacedevice 110. In some embodiments, the PCB 550 also supports a wirelesstransceiver directly, while in other embodiments the wirelesstransceiver is integrated with the controller 140. The indicator capassembly 540, body member assembly 544 and PCB 550 are mounted to a basemember 560, which is a molded plastic member in the embodiment of FIG.5D. The base member 560 secures the components of the user interfacedevice 110 to the exterior housing of the saw 100.

FIG. 3 depicts the user interface device 110 mounted on the exterior ofthe housing of the saw 100. The base member 560 attaches the componentsin the user interface device 110 to the exterior of the housing in thesaw 100 where the indicator lights 528A-528D are easily visible to theend user. Furthermore, the antenna 516 on the PCB 550 is positionedoutside of the electrical shielding of the saw 100, which provides botha clear view to enable communication with short-range external wirelessdevices and isolates the antenna 516 and any wireless transceivers onthe PCB 550 from sources of electrical noise within the saw 100. A datacable (not shown) connects the controller 140 mounted on the PCB withinthe housing of the saw 100 to the user interface device 110 on theexterior of the saw.

While the user interface device 110 depicted above includes lights and awireless data interface, in some configurations the saw 100 includesadditional data interface devices. For example, in one embodiment auniversal serial bus (USB) or other suitable wired data connector isoperatively connected to the controller 140. The saw 100 includes a USBport near the rear of the bevel carriage. The USB port is hidden fromordinary operators, but maintenance personnel access the USB port bymoving the bevel carriage to either the left or right extreme tiltposition and locating the USB port through an opening at the back of thehousing of the saw 100. The USB port is connected to an externalcomputing device to perform diagnostic and maintenance operations. TheUSB connection also enables maintenance personnel to update storedsoftware programs in the memory 142 that the controller 140 executesduring operation of the saw 100.

Referring again to the saw configuration of FIG. 2, in one operatingmode the controller 140 in the saw 100 employs an adaptive thresholdingprocess to identify the current spikes that correspond to the contactbetween an operator and the blade 108 to control operation of theimplement reaction mechanism 132. During the adaptive threshold process,the controller 140 identifies an average signal level for the sensingsignal over a predetermined period of time (e.g. 32 sampling periodsthat last 320 μsec at a sampling rate of 100 KHz). The controller 140applies a predetermined bias value to the detected average level anduses the sum of the average and the bias level as an adaptive threshold.The controller 140 updates the average threshold based on comparativelysmall changes in the average level of the sensing signal that occur dueto electrical noise, which prevents detection of a false positivecontact event when the level of the sensing signal only changes due toelectrical noise in the sensing signal. If contact between the operatorand the blade 108 occurs, the rapid spike in the sensing current exceedsthe predetermined bias level and the controller 140 detects the contactand activates the implement reaction mechanism 132.

In an optional embodiment of the adaptive threshold detection process,the controller 140 also identifies the signal to noise ratio (SNR) inthe sensing signal in response to detecting a spike in the sensingsignal current to further reduce the likelihood of false positivedetection. The controller 140 identifies the SNR with reference to amean value of the signal over a predetermined time window divided by thevariance of the signal level over the same time window. In oneconfiguration, the controller 140 performs a block computation processto reduce the computational complexity of identifying the SNR, whichenable the controller 140 to identify the SNR within the operationaltiming constraints for operation of the implement reaction mechanism132. In the block computation process, the controller 140 identifies themean values of the signal over comparatively short blocks (e.g. 32sampling periods that last 320 μsec at a sampling rate of 100 KHz) andstores the computed block mean values in a memory. The controller 140then identifies the SNR over a series of the blocks, such as eightconsecutive blocks of time over a period of 2560 μsec in one embodiment.

The controller 140 identifies a single variance value for all of theblocks based on the difference between the eight “local” mean valuesthat occur in each of the eight blocks and a single “global” averagemean value for all eight blocks. The controller 140 identifies the SNRbased on only the eight mean values an d the identified variance valueof instead of identifying the mean and variance over a total of 256separate samples. The block computation process greatly reduces thecomputational power that is required to identify the SNR. The controller140 continues identification of additional samples over time and updatesthe SNR sample after the oldest block is removed from the set of eightblocks to accommodate newer samples during operation. Afteridentification of the SNR, the controller 140 identifies if the SNRlevel is below a predetermined minimum threshold at the time ofdetecting a sensing current spike that exceeds the detection thresholdfor contact between the operator and the blade 108. If the SNR level istoo low, which indicates a weak signal level in comparison to thedetected noise level, then the controller 140 does not operate theimplement reaction mechanism 132 to prevent false positive operationwhen the operator is not actually in contact with the blade 108.

Another optional configuration of the adaptive thresholding processincludes an operation to detect static discharge from the blade 108 andprevent a static discharge event from being incorrectly identified ascontact between the operator and the blade 108. During operation of thesaw 100, the rotating blade may accumulate static and discharge thestatic to components within the saw 100 or to an external object such asa work piece. The static discharge often produces a momentary positiveor negative voltage spike in the sensing signal that is similar to thespike that occurs in response to contact between the operator and theblade 108. However, the amplitude of the spike due to static dischargeis often several times larger than any spike that is generated due tocontact with the operator. Consequently, in some embodiments thecontroller 140 identifies human contact not only in response to theamplitude of the sensing signal exceeding the adaptive threshold, butalso in response to the amplitude of the spike being below an upperbound threshold that is higher than the initial detection threshold toavoid a false-positive operation of the implement reaction mechanism 132in response to a static discharge event.

The adaptive thresholding process is useful in multiple operating modesof the saw 100 including operating modes in which the saw 100 performs“DADO” cuts. As is known to the art, during a DADO cut operation theblade 108 cuts a trench through all or a portion of a work piece, butdoes not completely cut the work piece into two separate parts. ManyDADO cuts produce trenches that are thicker than a single saw blade, andthe saw 100 operates with multiple saw blades placed together on thearbor 109 to form the thicker trenches. The multiple saw blades act asan antenna and receive electrical noise from various sources inside andoutside of the saw 100, which reduces the signal to noise ratio duringDADO cuts.

In some embodiments, the controller 140 also detects contact between theoperator and the blade 108 over a longer period of time during DADOcutting operations to account for the increased noise levels that arepresent in the detection signal. For example, in one configuration thecontroller 140 identifies a spike in the current level that exceeds theadaptive threshold for contact detection in a first sample period. In ahigh-noise environment, a noise spike may also produce the large spikethat exceeds the adaptive threshold level. However, a true contact eventproduces a relatively consistent spike in the current that remains abovethe threshold for several sampling periods (e.g. up to 10 periods at asampling rate of 100 KHz). The controller 140 identifies the change inthe spike level over multiple sampling periods. If the amplitude of thespike remains high and does not change level by a large amount overseveral sampling periods, then the controller 140 identifies that theblade 108 is in contact with the operator and activates the implementreaction mechanism 132. If, however, the controller 140 identifies largevariations in the level of the sensing current spike, then thecontroller 140 identifies that the changes in the sensing current aredue to noise and does not operate the implement reaction mechanism 132.Even with the longer detection period, the total detection and operationtime of the object detection system 102 occurs within a period of only afew milliseconds to maintain the effectiveness of the implement reactionmechanism 132.

The adaptive thresholding process improves the accuracy of contactdetection during the DADO cut. However, the adaptive thresholdingprocess is not strictly required for use during DADO cut procedures, andthe adaptive thresholding process is also effective for use in othermodes of operation of the saw 100.

During operation of the saw 100, the controller 140 optionally performsa fault detection process to identify faults in the cable that connectsthe sensor plate 120 or implement enclosure 118 to the detection system102. The controller 140 identifies hard faults, such as full breaks inthe cable, via a continuity test. So-called “soft faults” occur when thecable is at least intermittently connected, but the quality of theconnection does not enable the sensing signal to reach the sensor plate120 and for the controller 140 to detect the sensing current through thecapacitor 124. In one configuration, the controller 140 identifies softfaults prior to activation of the motor 112. The controller 140generates the sensing current through the sensing cable while the motor112 remains deactivated and the level of electrical noise in the saw 100is relatively low. If the amplitude or noise levels of the sensingsignal deviate from expected values by more than a predeterminedoperational tolerance threshold, then the controller 140 identifies asoft fault in the sensing cable. The controller 140 produces an errorsignal through the user interface device 110 and prevents activation ofthe motor 112 in response to detection of hard or soft faults in thesensing cable until the sensing cable is repaired or replaced.

In some embodiments, the saw 100 characterizes the capacitance levels ofdifferent operators through contact with a capacitive sensor at apredetermined contact location in the saw. For example, in oneembodiment the saw 100 includes a metal handle that registers thecapacitance, conductance, and other electrical properties of the hand ofan operator when the operator grips the handle. In other embodiments, acapacitive sensor is located in a rail or other surface of the saw 100that an operator contacts during typical operation of the saw 100. Thecontroller 140 receives sensor data corresponding to the electricalproperties of each operator and adjusts the blade contact detectionthresholds and other operating parameters to improve the accuracy ofblade contact detection for each operator.

In some embodiments, the saw 100 performs pattern detection with thesensing signal to identify the state of the blade 108 during operation.For example, in one embodiment the controller 140 identifies elements ofthe sensing signal that correspond to tooth strikes between the blade108 and a work piece. The controller 140 optionally uses a tachometer orother RPM sensor to identify the rotational rate of the blade 108, andthe controller 140 receives data corresponding to the size and number ofteeth on the blade 108 to identify an expected frequency of toothstrikes as the blade 108 engages the work piece. The controller 140 usesthe expected tooth strike frequency to assist in identification ofsensing signals that may correspond to contact between the operator andthe blade 108 or that merely corresponding to electrical noise that isproduced when a tooth strikes the work piece.

In some embodiments of the saw 100, the controller 140 stores identifiedprofiles of the sensing signal while the saw 100 cuts different types ofmaterial. For example, the saw 100 cuts through different varieties ofwood or pieces of wood with varying moisture levels to identify theamplitude and noise levels for the sensing signal that are detectedwhile cutting a plurality of different types of wood or other material.The profile generation process optionally occurs at a factory prior toshipment of the saw 100. During subsequent operation, the operatorprovides input to characterize the type of material that the saw 100will cut, and the controller 140 retrieves a stored profile of theexpected sensing signal parameters from a memory to assist inidentification of the expected sensing signal when cutting the workpiece.

FIG. 9A depicts another embodiment of an object detection sensors thatis suitable for use in conjunction with the object detection system 102in the saw 100 or in another saw embodiment. In FIG. 9A, the throatplate 119 incorporates capacitive sensors 904, 908, and 912. Each of thesensors 904, 908, and 912 are capacitive sensors that can detect thepresence of a human hand or other body part either in contact or closeproximity to the surface of the corresponding capacitive sensor due to achange in capacitance around the sensor. By contrast, a work piece suchas wood produces a much different change in capacitance to enable acontroller, such as the controller 140 depicted in FIG. 2, todistinguish the work piece from a human body part. The capacitivesensors 904-912 are arranged along the cut direction 920, whichcorresponds to the direction that a work piece travels as the blade 108cuts the work piece. The capacitive sensor 904 is arranged in an areaacross the front of the saw blade 108. The capacitive sensors 908 and912 are arranged conformal to the saw blade 108 on the left and righthand sides, respectively, as viewed from the front of the saw blade 108.

As depicted in FIG. 9A, each of the capacitive sensors 904-912 occupiesa predetermined region of the throat plate 119, such as the rectangularregions depicted in FIG. 9A or another geometric shape. In someembodiments, the capacitive sensors 904-912 detect not only the presenceof a human body part proximate to the corresponding sensor, but also alocation of the human body part over the surface of the sensor and avelocity and direction of movement of the human body part over time. Thethermoplastic throat plate 119 isolates the capacitive sensors 904-912from the blade 108, the surface of the table 104, and from othercomponents within the saw.

FIG. 9B depicts a process 950 for operation of the capacitive sensors904-912 in the saw 100. In the description below, a reference to theprocess 950 performing a function or action refers to an operation of acontroller to execute stored program instructions in association withother components in the saw to perform the function or action. Theprocess 950 is described in conjunction with the embodiment of FIG. 9Aand the saw 100 for illustrative purposes.

Process 950 begins as the saw 100 is activated and the motor 112 movesthe blade 108 to cut work pieces (block 954). During operation, thecapacitive sensors 904-912 generate the capacitive sensing signals todetect the presence of objects that are proximate to the surfaces of thecapacitive sensors 904-912 in the throat plate 119 around the blade 108(block 958).

If the controller 140 identifies a change in the capacitance level ofone or more of the capacitive sensors 904-912 based on a change in an RCtime constant of the capacitive sensing signal, then the controller 140detect the presence of an object, such as a work piece or human bodypart, in the region around the saw blade 108 prior to contact betweenthe object and the saw blade (block 962). For example, in someembodiments, the capacitive sensors 904-912 include capacitive sensingelements that form one plate in a capacitor and an electricallynon-conductive dielectric that covers the capacitive sensing element andcovers the surface of the capacitive sensors 904-912. An oscillator inthe capacitive sensors generates a time-varying capacitive sensingsignal using an RC circuit formed from the capacitive element in eachsensor and a predetermined resistor. As is known in the art, the RC timeconstant changes in response to a change in the size of the capacitanceC in the RC circuit, and the capacitive sensor or an external controldevice identifies contact with objects based on changes in thetime-varying signal. An object positioned over the surface one of thesensors 904-912 acts as the second plate in a capacitor and produces achange in the capacitance level of the sensor.

If the controller 140 identifies that there is no object proximate tothe capacitive sensors (block 962) or that a detected object produces aminimal change in capacitance that corresponds to a work piece but not ahuman body part (block 966) then the saw 100 continues operation to cuta work piece (block 970). Electrically conductive objects, such as afinger or other body part of a human operator, produce comparativelylarge changes in capacitance while electrically nonconductive objects,such a wood work pieces, produce small changes in the capacitance level.As described above, the characteristics of a work piece such as woodgenerate a change in capacitance in the sensors 904-912 that issufficiently distinct from a human body part to enable the controller140 to distinguish between the work piece and a human body part that isin close proximity to the capacitive sensors 904-912.

During process 950, if the capacitive sensors generate a signalcorresponding to a sufficiently large change in capacitance thatcorresponds to the presence of a hand or other body part in closeproximity to the capacitive sensors 904-912, then the controller 140generates a warning output, deactivates the motor 112, or activates theimplement reaction mechanism 132 prior to the object contacting theblade 108 (block 974). In a configuration where the detected object hasnot actually touched the blade but has moved within a predetermineddistance of the blade, the controller 140 deactivates the motor 112 toenable the saw blade 108 to come to a halt but does not engage theimplement reaction mechanism 132 unless the object actually contacts theblade as detected using the object detection system 102 described above.In other embodiments, if the capacitive sensors 904-912 detect an objectcorresponding to a human body part, the controller 140 generates awarning signal, such as a light that is visible to the operator on thetable 104, for the operator prior to deactivating the motor 112 oroperating the implement reaction mechanism 132. In some embodiments theobject detection system 102 operates the implement reaction mechanism132 if the object contacts the blade 108 prior to the blade 108 comingto a complete halt or prior to the object coming into contact with theblade 108.

In some embodiments of the process 950, the capacitive touch sensors904-912 each include a two-dimensional grid of sensing elements thatenable the touch sensors to generate multiple capacitive detectionsignals corresponding to the position within the two-dimensional regioncovered by each of the capacitive sensors. In some configurations, thecontroller 140 generates a warning signal if a human body part object isdetected at a first position that is over one of the sensors 904-912 butbeyond a first predetermined distance from the blade 108 and thendeactivates the motor 112 if the object moves with the predetermineddistance of the blade 108. Furthermore, the controller 140 or othercontrol device identifies a path of movement and velocity of the objectbased on a series of object locations that the individual sensingelements in the capacitive sensors 904-912 generate over time. If thepath of movement indicates that an object, such as a human hand, has ahigh likelihood of contacting the blade 108 at some point along thepath, then the controller 140 deactivates the motor 112 or generates thewarning output as described above. Additionally, in some configurationsthe controller 140 activates the implement reaction mechanism 132 toretract the blade 108 or other implement prior to actual contact betweenthe hand or other body part of the operator. For example, if thedetected location of a hand of the operator is within a predetermineddistance of the blade 108 or the path of movement of the hand over thecapacitive sensors has a trajectory that could with the blade 108, thenthe controller 140 optionally activates the implement reaction mechanism132 before the contact with the blade 108 actually occurs. Of course,the capacitive sensors 904-912 and the process 950 can be implemented intandem with the operation of the object detection system 102 that isdescribed above to detect the presence of a body part of the operator inproximity to the blade 108 and to detect actual contact between the bodypart and the blade 108.

In addition to the operation of the object detection system 102 that isdescribed above, the saw 100 is further configured to perform differentconfiguration and diagnostic processes to maintain reliability andenable operation of the saw with a wide range of different materials.For example, the saw 100 is configured to maintain a record of thenumber of times that the implement reaction mechanism has been activatedto ensure that the saw 100 receives proper maintenance.

FIG. 10 is a block diagram of a process 1000 for monitoring theoperation of the implement reaction mechanism in the saw. In thediscussion below, a reference to the process 1000 performing a functionor action refers to the operation of a controller to execute storedprogram instructions to perform the function or action in associationwith one or more components in the saw. The process 1000 is described inconjunction with the saw 100 for illustrative purposes.

Process 1000 begins upon activation of the implement reaction mechanism(block 1004). In the saw 100, the controller 100 activates the implementreaction mechanism 132 in response to detection of contact between anobject, such as the hand of the operator, other than a work piece. Inone embodiment of the saw 100, a pyrotechnic charge in the implementreaction mechanism 132 fires to retract the blade 108 below the level ofthe table 104. The controller 140 increments a counter held in anon-volatile portion of the memory 142 to maintain a record of thenumber of times that the implement reaction mechanism has been activatedduring operation of the saw 100 (block 1008). As is known to the art,the non-volatile memory, such as a solid-state or magnetic data storagedevice, retains stored data over a long period of time even when the saw100 is deactivated and disconnected from electrical power.

The process 1000 and the operation of the saw 100 continues while thetotal number of activations of the implement reaction mechanism 132remains below a predetermined threshold (e.g. five activations of theimplement reaction mechanism 132) (block 1012). If the number ofactivations of the implement reaction mechanism exceeds thepredetermined threshold (block 1012) then the controller 140 disablesoperation of the saw 100 until the saw 100 undergoes a maintenanceprocedure (block 1016). For example, in one configuration the controller140 ignores any input signals from the user interface 110 to activatethe saw 100, and the motor 112 remains deactivated while the saw 100 isdisabled. The controller 140 optionally generates an output indicatedsignal via the user interface 110 to alert the operator that the saw 100is disabled and requires maintenance.

The process 1000 continues during a maintenance operation. In additionto any required maintenance to repair or replace mechanical orelectrical components in the saw 100, the maintenance operation furtherincludes resetting the counter value in the memory of the saw 100 toreturn the saw to operation (block 1020). In one embodiment, themaintenance process includes connection of an external programmingdevice, such as a PC or other computerized programming device, to amaintenance port within the saw 100, such as a universal serial bus(USB) port, to both retrieve diagnostic data from the memory 142 andreprogram the memory 142 to reset the counter that stores the number oftimes that the implement reaction mechanism has been activated. The useof the external programming device enables the saw 100 to be re-enabledfor use after the maintenance process while remaining disabled until thesaw undergoes proper maintenance.

The process 1000 ensures that the saw 100 remains disabled untilreceiving maintenance in the event of an unusually large number ofactivations of the implement reaction mechanism 132. The maintenanceoperation ensures that all components within the saw 100 are operatingproperly and that the object detection system 102 accurately detectscontact between an object other than a work piece and the saw blade 108.

As described above, the object detection system 102 receives inputsignals in response to contact between the blade 108 and any object,including both work pieces which the saw cuts during normal operation,and other objects including potentially a body part of the saw operatorthat results in activation of the implement reaction mechanism. Duringoperation of the saw 100, the object detection system 102 receives inputsignals corresponding to changes in the capacitance levels in thecapacitor 124 formed by the plate 120 and blade 108 corresponding toboth contact between work pieces and potential contact with objectsother than work pieces. For example, in some situations wood with highmoisture content has the potential to be confused with a portion of thebody of a human operator during operation of the saw. FIG. 11 depicts aprocess 1100 that generates profiles of the signals generated bydifferent types of material in various work pieces to improve theaccuracy of object detection. In the discussion below, a reference tothe process 1100 performing a function or action refers to the operationof a controller to execute stored program instructions to perform thefunction or action in association with one or more components in thesaw. The process 1100 is described in conjunction with the saw 100 forillustrative purposes.

Process 1100 begins as the saw operates with the object detection system102 enabled but the implement reaction mechanism 132 disabled (block1104). The operation of the saw 100 without the implement reactionmechanism being enabled occurs under controlled conditions such as at afacility of the manufacturer or an approved maintenance facility. Duringthe process 1100, the saw cuts various materials in work pieces that aresuitable for use with the saw 100 but have the potential to producesensing signals that could be misinterpreted as corresponding to a humanbody part or other object that should trigger the implement reactionmechanism 132 upon contact with the spinning blade 108.

Process 1100 continues as the saw 100 records sensing signals in theobject detection system 102 that are produced at predetermined timeswhen the work piece initially contacts the blade 108, during the cut asthe work piece moves past the blade 108, and at the time of completionof the cut when the work piece disengages from the blade 108 (block1108). The recorded sensing signal information typically includes spikesin the sensing signal that correlate to changes in the capacitance levelin the capacitor 124. For example, the initial spike that occurs whenthe work piece initially contacts the rotating blade 108 may be similarto the initial spike that is generated when an object other than thework piece initially contacts the rotating blade 108.

In another embodiment of the process 1100, the saw 100 includesadditional sensors other than the capacitive sensor formed by thecapacitor 124 that can detect characteristics of the work piece materialthat can be distinguished from the body of a human operator. Forexample, one embodiment further includes one or more infrared sensorsthat are mounted on the riving knife 330 that is depicted in FIG. 3. Theinfrared sensors generate a profile of the frequency response ofinfrared light that is reflected from the work piece. The controller 140is operatively connected to the infrared sensors to record the frequencyresponse of the material in the work piece.

Process 1100 continues as the controller 140 or a processor in anexternal computing device identifies differences between the recordedsensing signal and the predetermined sensing signal profile for anobject that would trigger the implement reaction mechanism in the saw100 (block 1112). For example, as described above the controller 140uses an adaptive thresholding process to identify spikes in the sensingcurrent that correspond to a hand or other portion of the human bodywhen in contact with the blade 108. The spike corresponding to thecontact with the human hand includes both an amplitude profile and timeprofile. The controller 140 identifies differences in the amplitude andduration between the predetermined profile for a human body part and theinitial spike that occurs when the work piece first contacts therotating blade 108 and any subsequent spikes that occur as the blade 108cuts the work piece and disengages from the work piece.

The controller 140 or external processor then generates a detectionprofile that is specific to the test material based on the differencesbetween the recorded sense signals and the predetermined objectdetection profile for the human body (block 1116). In one embodiment,the controller 140 generates a profile with a range of amplitude valuesaround the amplitude of the recorded spike in the sensing signal whenthe blade 108 engages the predetermined material in the work piece. Therange of values for the amplitude of the spike does not include thethreshold amplitude for the spike amplitude for the predetermine profileof the human operator to ensure that the controller 140 does notincorrectly identify a sensing signal corresponding to a human operatoras a work piece. Thus, the size of the range of amplitude values thatcorrespond to different work pieces varies based on the differencesbetween the recorded spikes produced through contact between the blade108 and the work piece material and the predetermined profilecorresponding to a human body. The controller 140 similarly generates atime range corresponding to the duration of the spike in the sensingsignal from the work piece based on differences between the time rangeof the spike from the work piece and an expected duration of the spikein the profile for contact with a human body. The updated profileenables the controller 140 to distinguish between sense signals from thecapacitor 124 that correspond to contact between the blade 108 and awork piece of a predetermined type of material compared to potentialcontact with a portion of the human body.

As noted above, in an alternative embodiment the controller 140generates a profile based on the data from the infrared sensors toidentify the frequency response range of the material in the work pieceand distinguish the frequency response range from a predeterminedfrequency response range that is associated with a human operator. Thecontroller 140 uses a predetermined response range for the operator thatis stored in the memory 142 to ensure that the frequency response rangein the profile of the material does not overlap the predeterminedprofile for the human operator. For example, in one configuration thememory 142 stores a frequency response profile for near infraredresponses that have a peak response at wavelengths of approximately 1080nm and a minimum response at wavelengths of approximately 1580 nm for awide range of human skin tones. Other types of materials for variouswork pieces have peak and minimum infrared frequency responses atdifferent wavelengths, and the controller 140 generates a profile with arange of frequency responses for both peak and minimum response valuesat wavelengths that correspond to work pieces but do not overlap withthe wavelengths corresponding to the responses of human skin.

During process 1100, the updated profile for the test material is storedin the memory 142 (block 1120). During subsequent operations with boththe object detection system 102 and implement reaction mechanism 132being enabled, the controller 140 uses the stored profile informationfor the test material to reduce potential occurrences of false positivedetection events when changes in the sensing signal that occur due tocontact between the work piece and the saw blade 108 are misinterpretedas corresponding to contact between the operator and the saw blade. Forexample, if the saw 100 is cutting a particular type of material that isstored in a profile in the memory 142, then the controller 140 continuesto operate the saw 100 as long as any spikes in the sensing signal inthe object detection system 102 remain within the amplitude and timeduration ranges for the stored profile corresponding to the materialtype. In some configurations, the memory 142 stores profiles formultiple types of material that the saw 100 cuts during operation. Theoperator optionally provides an input to the saw 100 that specifies thetype of material to be cut to enable the controller 140 to use a storedprofile for the appropriate type of material in the work piece.

As described above, the object detection system measures changes in thesensing signal through the capacitor 124 in response to contact betweenobjects and the rotating saw blade 108. The memory 142 storespredetermined threshold information that the controller 140 uses withthe adaptive threshold process described above to detect contact betweenthe body of the human operator and the blade 108. However, the bodies ofindividual human operators may exhibit different capacitance levels bothbetween individuals and the capacitance level of one individual may varyover time for a variety of reasons. Examples of factors that affect thecapacitance levels of operators include, but are not limited to, thetemperature and ambient humidity in the environment around the saw, thephysiological makeup of each operator, the perspiration level of theoperator, and the like. FIG. 12 depicts a process 1200 for measuring thelevel of capacitance of an individual operator during operation of thesaw 100 to enable the saw 100 to adjust the object detection thresholdsfor different individuals. In the discussion below, a reference to theprocess 1200 performing a function or action refers to the operation ofa controller to execute stored program instructions to perform thefunction or action in association with one or more components in thesaw. The process 1200 is described in conjunction with the saw 100 forillustrative purposes.

Process 1200 begins as the saw 100 measures a capacitance level of theoperator through a capacitive sensor that is formed in a handle or otherpredetermined contact location on a surface of the saw 100 that theoperator touches during operation of the saw (block 1204). Using theillustration of FIG. 3 as an example, capacitive sensors in one or moreof the rip fence 304, the front rail 310, the bevel adjustment handle352, the height adjustment handle 354, or other surfaces of the saw thatthe operator touches during operation generate measurements of thecapacitance level in the hands of the operator. The operator does notneed to remain in continuous contact with the capacitive sensor duringoperation of the saw 100, but the controller 140 optionally updates themeasured capacitance level when the operator touches one or more of thecapacitive sensors.

Process 1200 continues as the controller 140 modifies the thresholdlevel for detection of contact with an object, such as the body of theoperator, other than the work piece (block 1208). The controller 140decreases the default spike amplitude detection threshold for thesensing signal in response to a measured capacitance level that is lessthan a predetermined default level, which can occur when the operatorhas unusually dry skin or other environmental factors reduce theeffective capacitance in the body of the operator. The controller 140modifies the threshold based on the difference between a defaultcapacitance level that is appropriate for a wide range of humanoperators and the measured capacitance level that may be higher or lowerthan the default level. Reducing the threshold level effectivelyincreases the sensitivity for detection between the operator and the sawblade 108 in the saw 100. The controller 140 optionally increases thethreshold in response to an identification of a large capacitance valuein the operator. In some embodiments the controller 140 limits themaximum threshold level for object detection to ensure that the objectdetection system 102 retains the capability to detect contact betweenthe operator and the blade 108 since increasing the detection thresholdlevel effectively reduces the sensitivity of the object detection system102.

Process 1200 continues as the saw 100 operates to cut work pieces andthe object detection system 102 uses the modified detection threshold todetect potential operator contact with the blade 108 (block 1212). Asdescribed above, if the hand or other body part of the operator contactsthe rotating blade 108, the controller 140 compares the amplitude of themeasured spike in the sensing signal through the capacitor 124 to themodified threshold using the adaptive thresholding process describedabove. Since the controller 140 modifies the detection threshold basedon the measured capacitance of the operator, the process 1200 enablesthe saw 100 to detect contact between the operator and the saw blade 108with improved accuracy.

In the saw 100, the motor 112 includes one or more brushes that engage acommutator. The use of brushes in electric motors is well-known to theart. Over time, brushes experience wear, which reduces the efficiency ofthe motor and worn brushes often generate sparks. The sparks can bedetrimental to operation of the motor 112 and in some circumstances thesparks generate electrical noise that is detected by the objectdetection system 102. FIG. 13A depicts an example of the shaft 1350,commutator 1354, and brushes 1358A and 1358B in the motor 112. Springs1362A and 1362B bias the brushes 1358A and 1358B, respectively, intocontact with the commutator 1354. In many embodiments, the brushes 1358Aand 1358B are formed from graphite. In the motor 112, the mounts 1366Aand 1366B are formed in a housing of the motor 112 and engage thesprings 1358A and 1358B, respectively. In one embodiment, the mounts1366A and 1366B include pressure sensors that measure the compressiveforce applied through the springs 1362A and 1362B. In anotherembodiment, the mounts 1366A and 1366B generate sensing currents throughthe springs 1362A and 1362B and the corresponding brushes 1358A and1358B to identify the electrical resistance levels through the brushes.

Since worn brushes not only reduce the efficiency of operation of themotor 112, but may introduce additional electrical noise into thesensing signal for the object detection system 102, the saw 112optionally detects brush wear in the motor 112 and generates an outputto indicate that worn brushes should be replaced via the user interface110. FIG. 13B depicts a first embodiment of a process 1300 for measuringbrush wear in the motor 112. In the description below, a reference tothe process 1300 performing an action or function refers to theoperation of a controller, such as the controller 140 in the saw 100, toexecute stored program instructions to perform the function or action inconjunction with other components in the saw 100.

During process 1300, an electrical source positioned in each of themounts 1366A and 1366B generates an electrical current through thecorresponding brushes 1358A and 1358B (block 1304). In one embodiment,the current passes through the cables that are connected to the brushes1358A and 1358B for normal operation of the brushes 1358A and 1358B inthe saw 100. In another configuration, the current passes through thesprings 1362A and 1362B and the corresponding brushes 1358A and 1358B.The current is generated during a diagnostic mode when the saw motor 112is otherwise deactivated and the level of the electrical current used inthe process 1300 is well below a drive current that produces rotation inthe motor shaft 1350 during operation of the motor 112. During process1300, the controller 140 or a controller that is integrated with themotor 112 measures an electrical resistance level through the brushesand compares the measured electrical resistance level to a predeterminedresistance threshold (block 1308). The measurement of the electricalresistance level includes, for example, a measurement of a voltage levelor current level for the electrical current that flows through each ofthe brushes 1358A and 1358B in the diagnostic mode and an application ofOhm's law to find the resistance (e.g. R=E/I for a measured voltage Eand predetermined current I or predetermined voltage E and measuredcurrent I). Once the resistance drops below a predetermined threshold,the controller 140 generates an output signal via the user interface 110to indicate that the brushes should be replaced (block 1312). Theresistance drops as the brushes wear and grow thinner, which reduces thetotal resistance through the springs 1362A and 1362B and thecorresponding brushes 1358A and 1358B. In some configurations, thecontroller 140 also disables operation of the saw 100 until any wornbrushes have been replaced and the controller 140 performs the process1300 again to confirm that the new brushes are no longer worn.

FIG. 13C depicts a second embodiment of a process 1320 for measuringbrush wear in the motor. In the description below, a reference to theprocess 1320 performing an action or function refers to the operation ofa controller, such as the controller 140 in the saw 100, to executestored program instructions to perform the function or action inconjunction with other components in the saw 100.

In the process 1320, the spring mounts 1366A and 1366B each include apressure sensor that measures the compressive force of the correspondingsprings 1362A and 1362B during a diagnostic mode when the motor 112 isdeactivated (block 1324). As the brushes 1358A and 1358B experiencewear, the corresponding springs 1362A and 1362B expand to bias thebrushes onto the commutator 1354. The compressive force in the springs1362A and 1362B decreases as the springs expand. The controller 140 or acontroller in the motor 112 is operatively connected to the pressuresensors and compares the measured pressure levels from the pressuresensors to a predetermined pressure threshold (block 1328). Once thepressure sensors in the mounts 1366A and 1366B measure that thecompressive force of the springs 1362A and 1362B has dropped below apredetermined threshold, the controller 140 generates an output signalvia the user interface 110 to indicate that the brushes should bereplaced (block 1332). In some configurations, the controller 140 alsodisables operation of the saw 100 until any worn brushes have beenreplaced and the controller 140 performs the process 1320 again toconfirm that the new brushes are no longer worn.

As described above, during operation the object detection system 102receives sensing signals through a single sensing cable, such as thecoaxial cable 720 depicted in FIG. 8B, that includes two differentconductors. Within a high vibration environment such as the saw 100, thesensing cable 720 may experience wear and faults over time thateventually require cable replacement during saw maintenance. If thesensing cable 720 breaks or is disconnected from any of the PCB of theobject detection system 102, the plate 120, or implement enclosure 118,then the PCB does not detect any sensing signal and can disable the saw100 until the single sensing cable 720 is repaired. However, in somecircumstances the sensing cable 720 experiences a “soft fault” in whichthe cable is not completely disconnected, but the cable continues tooperate with greatly degraded performance in the saw. The PCB 102 stillreceives the sensing signal, but the fault within the sensing cable 720introduces noise or attenuates the sensing signal, which reduces theaccuracy of the object detection system 102. FIG. 14 depicts a process1400 for diagnosing soft faults in the sensing cable 720. In thedescription below, a reference to the process 1400 performing an actionor function refers to the operation of a controller, such as thecontroller 140 in the saw 100, to execute stored program instructions toperform the function or action in conjunction with other components inthe saw 100.

Process 1400 begins as the object detection system 102 generates apredetermined excitation signal during a diagnostic mode (block 1404).In one embodiment, the controller 140 activates the clock source 144 togenerate the same sinusoidal sensing signal that is used duringoperation of the saw 100 using amplitude modulation. In anotherembodiment, the clock source 144 produces an impulse train including aseries of delta pulses at a predetermined frequency to enable thecontroller 140 to receive an output corresponding to the unit impulseresponse of through the sensing cable 720 and capacitor 124. In furtherembodiments, the clock source 144 generates any suitable predeterminedsignal that enables diagnosis of potential faults within the sensingcable 720. During the diagnostic mode, the motor 112 in the saw 100 isdeactivated and there is minimal electrical noise present within thesaw.

Process 1400 continues as the controller 140 identifies a signal tonoise ratio (SNR) of the detected excitation signal (block 1408). In thesaw 100, the controller 140 detects a return signal in response to theexcitation signal from the clock source 144 and the amplifier 146 thatpasses through the sensing cable 720 and the plate 120 and saw blade 108of the capacitor 124. Since the clock source 144 and driving amplifier146 generate the excitation signal with a predetermined amplitude andmodulation, the controller 140 identifies the SNR using a predeterminedmeasurement technique that is otherwise known to the art. Of course,even in a an otherwise deactivated saw the excitation signal experiencessome degree of attenuation through the sensing cable 720 and capacitor124, and some degree of noise, such as Johnson-Nyquist noise, is alwayspresent within the sensing circuit. As used in the context of theprocess 1400, a measurement of SNR also includes a measurement of signalstrength attenuation that does not include a direct measurement ofnoise. For example, the predetermined excitation signal is generatedwith a predetermined amplitude and the controller 140 measures theamplitude of the return signal. Some level of attenuation in the returnsignal is expected and a predetermined amplitude level for the signalstrength of the return signal for a properly functioning sensing cableis identified empirically and stored in the memory 142. However, if theamplitude of the return signal drops below a predetermined level, thenthe controller 140 identifies a potential fault in sensing cable 720.

In an alternative configuration, the sensing cable 720 includes a thirdconductor that is electrically isolated from the first conductor andsecond conductor in the sensing cable. In one embodiment, the thirdconductor is formed as part of a second twisted pair in the sensingcable 720, while in another embodiment the sensing cable includes twocoaxial elements that form three separate conductors. One end of thethird conductor is connected to the plate 120 in a similar manner to thefirst conductor as depict in FIG. 8C. The other end of the thirdconductor is connected to an analog to digital converter (ADC) that ismounted to the PCB of the object detection system to provide a digitizedversion of the sensing signal to the controller 140. During the process1400, the controller 140 measures a return signal based on theexcitation signal through the third conductor instead of through thefirst conductor and the second conductor.

The controller 140 identifies if the measured SNR for the excitationsignal drops below a predetermined minimum SNR ratio that is suitablefor operation of the object detection system 102 (block 1412). A faultin the sensing cable 720 attenuates the level of the received signal,introduces additional noise into the sensing cable 720, or produces bothan attenuation of signal strength and increase in noise that degradesthe SNR. If the SNR remains above the predetermined threshold, then thesensing cable 720 is considered functional and the saw 100 continueswith operation (block 1416). However, if the measured SNR falls belowthe predetermined threshold, then the controller 140 generates an outputindicating a potential fault in the sensing cable (block 1420). In thesaw 100, the controller 140 generates the output via the user interface110 to alert an operator to a potential cable fault. In someconfigurations, the controller 140 disables operation of the saw 100until the sensing cable 720 is repaired or replaced.

It will be appreciated that variants of the above-described and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems, applications or methods.Various presently unforeseen or unanticipated alternatives,modifications, variations or improvements may be subsequently made bythose skilled in the art that are also intended to be encompassed by thefollowing claims.

What is claimed:
 1. A detection system for detecting contact between animplement in a saw and an object comprising: an electrically conductiveplate positioned at a predetermined distance from the implement; adetection circuit comprising a transformer, the transformer comprising afirst winding formed from a first electrical conductor having a firstterminal and a second terminal; a single cable connecting the firstwinding to the electrically conductive plate and the implement, thesingle cable comprising: a first conductor electrically connected to thefirst terminal of the first winding and to the electrically conductiveplate; a second conductor electrically connected to the second terminalof the first winding and to the implement; and an electrical insulatorpositioned between the first conductor and the second conductor; animplement enclosure; and an arbor connected to the implement enclosureand the implement, wherein the second conductor is electricallyconnected to the implement through the implement enclosure and thearbor, wherein the implement enclosure includes: a height adjustmentcarriage; and a bevel carriage, and wherein the second conductor iselectrically connected to the height adjustment carriage in a firstlocation and is electrically connected to the bevel carriage in a secondlocation.
 2. The system of claim 1, wherein: the first conductor is acentral conductor in a coaxial cable, the second conductor is a ribbonconductor in the coaxial cable surrounding the first conductor, and theelectrical insulator is arranged between the first conductor and thesecond conductor in the coaxial cable.
 3. The system of claim 1,wherein: the transformer further comprises a second winding formed froma second electrical conductor having a third terminal and a fourthterminal; and the detection circuit further comprises: a firstdemodulator electrically connected to the third terminal of the secondwinding; a second demodulator electrically connected to the fourthterminal of the second winding; a clock generator electrically connectedto the first winding and configured to generate a sensing signal throughthe first winding at a predetermined frequency; and a controllerconfigured to receive an in-phase signal from the first demodulator anda quadrature-phase signal from the second demodulator.
 4. The system ofclaim 3, further comprising: an implement reaction mechanism operativelyconnected to the implement, wherein the controller is operativelyconnected to the implement reaction mechanism and is further configuredto: identify a spike in the sensing signal with reference to thein-phase signal from the first demodulator and the quadrature-phasesignal from the second demodulator; and generate a control signal tooperate the implement reaction mechanism in response to theidentification of the spike.
 5. The system of claim 1, furthercomprising: a metal sleeve mounted to the implement enclosure andsurrounding a portion of the second conductor to establish theelectrical connection between the second conductor and the implementthrough the implement enclosure.
 6. The system of claim 1, wherein: thefirst conductor is a first conductor in a single twisted pair cable, thesecond conductor is a second conductor in the twisted pair cable, andthe electrical insulator separates the first conductor and the secondconductor in the twisted pair cable.
 7. The system of claim 6, thetwisted pair cable further comprising: a metallic shield surrounding thefirst conductor, the second conductor, and the electrical insulator. 8.The system of claim 1, further comprising: a table including an openingfor the implement, the table electrically isolated from the implementand the electrically conductive plate; and a first electrical cableconnecting the table to an electrical ground.
 9. A detection system fordetecting contact between an implement in a saw and an objectcomprising: an electrically conductive plate positioned at apredetermined distance from the implement; a detection circuitcomprising a transformer, the transformer comprising a first windingformed from a first electrical conductor having a first terminal and asecond terminal; a single cable connecting the first winding to theelectrically conductive plate and the implement, the single cablecomprising: a first conductor electrically connected to the firstterminal of the first winding and to the electrically conductive plate;a second conductor electrically connected to the second terminal of thefirst winding and to the implement; and an electrical insulatorpositioned between the first conductor and the second conductor; a firstprinted circuit board (PCB) configured to support the detection circuit;a second PCB configured to support a power supply and a TRIAC; a datacable operatively connected to the first PCB and the second PCB toenable the detection circuit to transmit a control signal from the firstPCB to the second PCB; and a ferrite choke formed around the data cable.10. The system of claim 9, further comprising: a tamp resistorpositioned on the first PCB and connected to the data cable and anelectrical ground on the first PCB.
 11. A detection system for detectingcontact between an implement in a saw and an object comprising: anelectrically conductive plate positioned at a predetermined distancefrom the implement; a detection circuit comprising a transformer, thetransformer comprising a first winding formed from a first electricalconductor having a first terminal and a second terminal; a single cableconnecting the first winding to the electrically conductive plate andthe implement, the single cable comprising: a first conductorelectrically connected to the first terminal of the first winding and tothe electrically conductive plate; a second conductor electricallyconnected to the second terminal of the first winding and to theimplement; and an electrical insulator positioned between the firstconductor and the second conductor; a table including an opening for theimplement, the table electrically isolated from the implement and theelectrically conductive plate; a first electrical cable connecting thetable to an electrical ground; a second cable electrically connected toan implement enclosure and to the electrical ground through a firstresistor; and a third cable electrically connected to the implement andto the electrical ground through a second resistor.
 12. The system ofclaim 11, wherein the first resistor and the second resistor each havean electrical resistance level of approximately 1 MΩ.
 13. A detectionsystem for detecting contact between an implement in a saw and an objectcomprising: an electrically conductive plate positioned at apredetermined distance from the implement; a detection circuitcomprising a transformer, the transformer comprising a first windingformed from a first electrical conductor having a first terminal and asecond terminal; a single cable connecting the first winding to theelectrically conductive plate and the implement, the single cablecomprising: a first conductor electrically connected to the firstterminal of the first winding and to the electrically conductive plate;a second conductor electrically connected to the second terminal of thefirst winding and to the implement; and an electrical insulatorpositioned between the first conductor and the second conductor; a tableincluding an opening for the implement, the table electrically isolatedfrom the implement and the electrically conductive plate; a firstelectrical cable connecting the table to an electrical ground; and a ripfence positioned over a surface of the table, the rip fence including afirst electrical insulator positioned between the rip fence and thetable and a second electrical insulator positioned above the surface ofthe rip fence.